In order to have at least 80% probability to demonstrate a statistically significant difference in average AL between two treatment groups, given that the true difference is at least 1.0 mm, at least 36 patients must be available for statistical analysis in each group. We expect that about 10% of the patients will drop out of the study during the 5 years follow-up period, but we cannot exclude the possibility of a drop-out rate as high as 20%.